The determination of density of cryogens, such as liquified gases and associated solids, gases and mixtures thereof, is a difficult problem because cryogens are volatile and measurements must take place at cryogenic temperatures.
U.S. Pat. No. 3,933,030 discloses a system of taking continuous simultaneous, or sequential, readings of density in a cryogenic tank to monitor density changes and thereby create a density profile of the content of the tank to avoid the occurrence of rollover, or density inversions which, when left unchecked, can create a sudden emission of large quantities of highly flammable and explosive gases. U.S. Pat. No. 3,933,030 suggests that the density of cryogenic liquids can be continuously determined electrically by the use of differential capacitance measurements. Density is determined from the Clausious-Mosotti equation by determining the dielectric constant of the cryogenic liquid with a dielectric cell of known capacitance, measuring the temperature of the cryogenic liquid and calculating the density of the cryogenic liquid from the Clausious-Mosotti equation.
Many systems have been disclosed for determining the dielectric constants of fluid materials. For example, U.S. Pat. No. 4,555,661 discloses method and apparatus for determining a dielectric constant of fluid materials through a differential capacitance technique. The disclosed apparatus of U.S. Pat. No. 4,555,661 consists of a three electrode device which may be placed in a container of fluid material. The electrodes of the device may be three planar electrodes or three coaxial cylindrical electrodes. The two remote electrodes are connected across the terminals of a free-running oscillator and determine its frequency of oscillation by their area and their separation and the dielectric constant of the material between the electrodes. The intermediate electrode is connected to a switch which alternatively connects the intermediate electrode to one of the remote electrodes and disconnects the intermediate electrode from the circuit thereby changing the frequency of oscillation. Dielectric constant is determined from the frequency difference.
U.S. Pat. No. 3,903,478 discloses a fluid density measuring device including fluid capacitance test cell comprising three coaxial conductive cylindrical electrodes surrounded by a protective outer tube. The test cell capacitor is formed by inner and outer concentric cylinders, which are interconnected, and the intermediate concentric cylinder and is connected with a 16 kHz precision oscillator to provide a current through the test cell capacitor that is proportional to the density of the fluid passing through the test cell. The 16 kHz precision oscillator also provides a current through another capacitor which is 180.degree. out-of-phase, and both currents are directed to an amplifier which provides an output proportional to the difference in the currents and therefore the density of the fluid through the test cell.
U.S. Pat. No. 4,544,880 discloses a microwave probe for measuring the amount of contaminants or water in crankcase oil, comprising a coaxial cable having a tip including five substantially parallel wires shorted together at one end and connected at the other end to the coaxial cable. The microwave probe includes a central wire connected to the center wire of the coaxial cable and four outer wires connected to the sheath of the coaxial cable at one end and connected to each other and to the center wire at the other end. The size and geometry of the wires is selected so that the impedance of the tip, when immersed in oil, substantially matches the impedance of the coaxial cable. The dielectric constant of the oil and the contaminant content of the oil is measured by connecting the microwave probe to a microwave oscillator and measuring either the voltage level of the standing wave at a fixed position along the interconnecting coaxial cable, or the null location of the standing wave along the interconnecting coaxial cable, or the operating frequency of the microwave oscillator required to keep the standing wave in a fixed position along the interconnecting coaxial cable.
Other systems which have been disclosed for the measurement of dielectric constants of fluids include those disclosed in U.S. Pat. Nos. 3,375,716; 3,739,266; 4,011,746; 4,417,472; 4,429,272; 4,468,611; 4,673,869; and 4,751,476.
U.S. Pat. No. 3,375,716 discloses a multivibrator driven with a fluid cell capacitor as a sensor element in the timing circuit of the multivibrator, to provide a liquid level gauge by varying the frequency of the multivibrator as a result of variations in capacitance of the sensor as liquid fills the fluid cell capacitor.
U.S. Pat. No. 3,739,266 discloses a system for protecting the moisture content of material such as cottonseed by the variation in the dielectric constant of a flow of such material past capacitor plates. The capacitor plates of U.S. Pat. No. 3,739,266 are connected to an oscillator generating an electrical signal whose frequency is proportional to the dielectric constant of the material.
U.S. Pat. No. 4,011,746 discloses a liquid density measurement system comprising a capacitance probe and a temperature sensor both immersible in a liquid whose density is to be measured. This system is particularly suited to measuring the density of liquid mixtures such as liquid natural gas, which is a non-polar organic material that follows the Clausious-Mosotti equation establishing the relationship between dielectric constant and density. The signals derived from the compacitance probe and temperature sensor are scaled and used to calculate density pursuant to the Clausious-Mosotti equation.
U.S. Pat. No. 4,417,472 discloses a liquid level sensor comprising four elongated L-shaped conductive capacitor elements arranged in a cruciform configuration. The four L-shaped conductive elements are sandwiched with a material of known dielectric constant to comprise one solid cross of the cruciform and to provide two open capacitor-forming spaces in the other cross of the cruciform that receive varying levels of fluid, thereby forming two capacitors of variable capacitance depending upon the level of the fluid in the spaces.
U.S. Pat. No. 4,429,272 discloses an apparatus for detecting the presence of water in fuel for an internal combustion engine by the change in dielectric constant of the fuel detected by two electrodes immersed in the fuel.
U.S. Pat. No. 4,468,611 discloses a four cavity fluid cell for use in a Wheatstone bridge configuration to equalize the effects of fluid characteristics, pressure and temperature in the measurement of the dielectric constant of a sampled fluid. U.S. Pat. No. 4,468,611 discloses a plurality of configurations forming four fluid capacitance-determining cells for connection into the Wheatstone bridge, including four L-shaped elongated conductive capacitive elements arranged in a cruciform configuration and supported by a central dielectric member and a surrounding dielectric member to form four capacity cells in the arms of the cruciform. One of the opposing pairs of open cells is provided with reference fluid and the other opposing pair of fluid cells is provided with sample fluid whose dielectric constant is to be determined.
U.S. Pat. No. 4,673,869 discloses a circuit for matching of the capacitance of the sample and reference cells from outside the oven of a chromatographic analyzer system. The circuit includes a dielectric constant detector including a detection cell providing a sample cell formed by a pair of cylindrical electrodes spaced apart along a common axis and surrounded by a common electrode. When the same fluid is going through both the sample and reference cell, a voltage applied to the varactor diode is manipulated until such time as the capacitance of the sample and reference cell are substantially matched.
U.S. Pat. No. 4,751,476 discloses a device and method for distinguishing between different fluids on the basis of their dielectric properties. The disclosed device includes two substantially parallel conductive surfaces that are positioned on opposite sides of a fluid conduit to form a substantially parallel plate capacitor. One of the conductive surfaces is connected through an inductor to a voltage source and the other of the conductive surfaces is connected to a resonator device. A transconductance amplifier is connected between the resonator device and the first conductive surface. The system provides oscillation when a known fluid passes between the parallel conductive plates but no oscillation when the fluid having different electrical dielectric properties passes between the plates because of the change of the capacitance due to differing dielectric constants.
Notwithstanding this prior inventive activity, a satisfactory density sensor for cryogens such as slush hydrogen and the like remained unavailable prior to this invention.